Communication cables have evolved continuously over the years as we have evolved from a voice-based telecommunication network environment to the new structured cabling designs for high-speed data transmission which are commonly referred to as Local Area Networks or LAN's. Technical requirements, standards and guidelines of the Telecommunication Industry Association and Electronic Industry Association (TIA/EIA) and International Standard Organization (ISO) have been developed and published to support high-speed data communication of voice, internet and video. In addition, these requirements continue to evolve with more and more stringent electrical performance needs such that cellular foam insulation and fillers play an increasing role in the cable designs. The primary communications cable designs incorporate twisted copper pairs together to form a balanced transmission line, coaxial cables, and fiber optic cables. All of these cables may be run in a network of a building (LAN's) as separate functional cables or in hybrid or combination cable design.
Furthermore, TIA/EIA has defined standards that are published and recognized as well as industry drafts of soon-to-be published standards for commercial building telecommunication networks. Table 1, which follows, provides those published and pending, or soon-to-be adopted and published Technical Service Bulletin “TSB” standards.
TABLE 1TIA/EIA StandardsCategory 5eFrequency BandwidthANSI/TIA/EIA-568-AISO Class D1 to 100 mhzCommercial BuildingTelecommunications StandardPart 2:Balanced Twisted Pair CablingComponent; 2001Category 6Frequency BandwidthANSI/TIA/EIA-568-B.2-1ISO Class E1 to 250 mhzCommercial BuildingTelecommunications StandardPart 2:Addendum 1: TransmissionSpecification for 4 pair 100 ohmCategory 6 Cabling; 2002Category 6AFrequency BandwidthANSI/TIA/EIA-568-B.2-10ISO Class EA1 to 500 mhzCommercial BuildingTelecommunications StandardPart 2:Addendum 10: TransmissionSpecification for 4 Pair 100 ohmAugmented Category 6 Cabling;TSB pending publicationCategory 7Frequency BandwidthTIA not actively developingISO Class F1 to 600 mhzstandard;ISO/EIA-11801, 2nd Ed.Information Technology -Generic Cabling for CustomerPremises, 2002
Each of the standards of Table 1 illustrate continued widened bandwidth enabling greater data transmission. The broadening of communication cable bandwidth enhances the electrical characteristics or data bit rate based on the evolving needs of software, hardware and video transmission. The terminology within the standards for testing can be defined as electrical performance within the cable as measured by impedance, near end and far end crosstalk (NEXT & FEXT), attenuation to crosstalk ratio (ACR), ELFEXT, ELNEXT, Power Sum, etc., and the electrical performance that may be transferred to the adjacent cable a.k.a. (alien cross talk) which are measured within similar performance parameters while incorporating a power sum alien cross talk requirement.
Electromagnetic noise that can occur in a cable that runs alongside one or more cables carrying data signals can create alien crosstalk. The term “alien” arises from the fact that this form of Crosstalk occurs between different cables in a group or bundle, rather than between individual wires or circuits within a single cable. Alien Crosstalk can be particularly troublesome because of its effect on adjacent 4 pair cables which degrades the performance of a communications system by reducing the signal-to-noise ratio.
Traditionally, alien crosstalk has been minimized or eliminated by aluminum MylarR shields and/or braid in shielded cable designs i.e. (Category 7 or ISO Class F shielded designs) to prevent electromagnetic fields from ingress or egress from the cable or cables. The use of foamed or blown constructions for symmetrical and asymmetrical airspace designs further improve electrical performance characteristics in that the overall modulus and elasticity of the resulting foamable compounds are reduced leading to final conformations that more closely approach optimal geometries. Specifically, the ability to form inner structures of cables such that these inner structures have little or no plastic memory once the cabling process is completed, ensures that the nested pairs remain in the desired geometric configuration and that the use of foamed fillers, insulations and jackets using air as an insulator act to mitigate alien crosstalk in Unshielded Twisted Pair (UTP) designs i.e. (Category 6 or ISO Class E and Category 6 Augmented or ISO Class EA).
Further developments have also recently embraced the cable fillers components, designed either to prevent cross-talk or alien cross-talk. As the TIA electrical requirements have moved from Augmented Cat. 6 to Augmented Cat. 6a, the electrical requirements also of the cable fillers materials are becoming more and more critical. Therefore the cable fillers are moving to perfluoropolymers such as FEP and TFE/perfluoroalkylvinylether copolymers to meet the attenuation requirements. As previously mentioned, the cable fillers' (like cross-webs') main function is to prevent cross-talk between the pairs.
More and more cables (for instance for data transmission between computers, voice communications, as well as control signal transmission for building security, fire alarm, and temperature control systems) are often installed in the air return space above the suspended ceiling without the use of metal conduits. Combustible materials (insulating or jacketing materials as well as fillers from cables) are thus accumulating in plenums in ever-increasing amounts.
While solutions for implementing physical foaming have been largely proposed in the past, this technology requires complex extrusion lines and accurate control of feeding of foaming agents. Standard equipments of LAN cables manufacturers cannot be efficiently retrofitted to produce foamed components by physical foaming. Intensive capital expenditure and increased operational costs are thus related to the implementation of physical foaming technologies. Moreover, fine cells are difficult to be obtained in foamed materials by physical foaming.
U.S. Pat. No. 6,064,008 discloses communication cables is provided having at least one elongate electrical conductor surrounded by a layer of insulating material, said layer including a chemically blown fluorinated polymer having a melting point of greater than about 480° F. The fluorinated polymer is preferably a high melting fluorinated polymer and is chemically blown by a blowing agent such as the barium salt of 5-phenyltetrazole. Nevertheless, this process requires the use of highly costly chemical blowing agent; also, due to the low thermal stability of the same, accurate control of processing is required for obtaining suitable foaming of the fluoropolymers having high melting point.
The designers of the first plenum cables used perfluoropolymers, the best available fire resistant and low smoke producing materials at the time. The first plenum cables listed had Fluorinated Ethylene Propylene (FEP) insulation and jackets, basically the same materials used today to meet the NFPA 262 or the limited combustible listing requirements of 25/50/8. Nevertheless, electrical performances of FEP are limited: even if bare FEP could be used, materials with improved electrical performance are desired to give more freedom with their design and give companies cables with a competitive advantage exceeding the minimum standards. Actually FEP bare or virgin material is hardly suitable for the manufacture of all parts of cables such as those for Gigabit Ethernet and future higher speed LAN applications, e.g. those complying with transmission requirements of ANSI/TIA/EIA 568-B.2, Addendum 1, Category 6 (so-called “category 6-cables”). Use of recycled FEP and other polymers for wire and cable and filler materials thereof can be utilized to improve properties and mitigate costs.
These Electrical Performance Standards especially for UTP cables (Category 5e, 6, 6A and 7) necessitate improved insulative performance wherein foamed perfluoropolymers optimize their inherently excellent insulative values i.e. (dielectric constant and dissipation factor.)
Foamed perfluoropolymers also offer lower cost and lower material content while improving fire retardancy performance by lowering the amount of combustible material in a cable and the overall fire load of Local Area Network cables within a building.
The Applicant has demonstrated that melt-processable per(halo)fluoropolymers, including FEP, PFA and MFA, can be compounded into pellets and subsequently chemically foamed via an extrusion process. The resulting foamed extrudate therefrom, in at least certain embodiments, would comply with the above-mentioned fire and smoke requirements and with sheathing requirements for next generation LAN cable.
A brief review of the Fire Performance Requirements both in North America and Globally follows:
In 1975, the National Fire Protection Agency (NFPA) recognized the potential flame and smoke hazards created by burning cables in plenum areas, and adopted within the United States, the National Electric Code (NEC), a standard for flame retardant and smoke suppressant cables. The National Electrical Code presently requires that such cables be either placed within a metal conduit or be insulated with materials that result in low flame and low smoke emission. This standard, commonly referred to as “the Plenum Cable Standard”, was later adopted for North America Communications Cabling by Canada and Mexico. The standard permits the use of power-limited type cables that includes communication cables without conduit, so long as the cable exhibits low smoke and flame retardant characteristics.
The premise of the standard is based on the concerns that flame and smoke could travel along the extent of a building plenum area if the electrical conductors and cable were involved and were not flame and smoke resistant. The National Fire Protection Association (“NFPA”) developed the standard to reduce the amount of flammable material incorporated into insulated electrical conductors and jacketed cables. Reducing the amount of flammable material would, according to the NFPA, diminish the potential of the insulating and jacket materials from spreading flames and evolving smoke to adjacent plenum areas and potentially to more distant and widespread areas within a building. The cellular foam fluoropolymer products of this disclosure can typically reduce the quantity of combustible materials by 30 to 60% based on the extent of the foaming process within insulations, fillers and jacket materials.
Nevertheless, all these designs require the development of improved fluoropolymer materials as cable fillers components having improved electrical properties, and still possessing adequate flammability properties. In order to meet the ever more stringent fire standards (NFPA 262 and/or “LC”) and to achieve electrical performances for novel LAN structures (Cat. 6 or 6a), it would be useful to find alternative insulation solutions which can provide improved properties both for reducing cross-talk and smoke/flame generation, and which enable achievement of these targets with potential reduction in cost. It has been thus proposed to use perfluoromaterials under the form of foams, so as to effectively reduce the mass of potentially combustible material while maximizing insulation and shielding performances. Basically, foaming of a perfluoropolymer can be obtained by physical foaming, i.e. by introduction of a suitable foaming agent (i.e. an inert gas) in the extrusion line processing the material in the melt state, or by chemical foaming, ie. by extrusion of a composition comprising suitable precursors undergoing thermal decomposition at processing temperatures so as to in situ generate the foaming agent required to form the cellular structure.
The accumulation of combustible materials in air return plenum spaces has caught the attention of American associations like the National Building Code Community, the National Fire Protection Association (NFPA) and two key groups within the NFPA (the 90A Heating and Ventilating Group, and the 70 National Electrical Code Group) because cables can present a larger fire load than wall coverings or furniture. NFPA 262 standard prescribes the methodology to measure flame travel distance and optical density of smoke for insulated, jacketed, or both, electrical wires and cables and optical fiber cables that are to be installed in plenums and other spaces used to transport environmental air without being enclosed in raceways.
The test method for measuring these characteristics is commonly referred to as the Steiner Tunnel Test. The Steiner Tunnel Test has been adapted for the burning of cables according to the following test protocols: NFPA 262, Underwriters Laboratories (U.L.) 910, or Canadian Standards Association (CSA) FT-6. The test conditions for each of the U.L. 910 Steiner Tunnel Test, CSA FT-6, and NFPA 262 are as follows: a 300,000 BTU/hour flame is applied for 20 minutes to a calculated number of cable lengths based on their diameter that fills a horizontal tray approximately 25 feet long with an enclosed tunnel. This test simulates the horizontal areas (ceilings) in buildings wherein these cables are run.
The pass/fail criteria require the cables to possess in the standardized Steiner tunnel test an Average Optical Density (AOD) (i.e. smoke) of <0.15, a Peak Optical Density (POD) (i.e. smoke) of <0.5 and a Flame Propagation Distance (FPD) of 5<ft. Further, even more stringent requirements have been settled for plenum permanent building materials so as to comply with the “Limited Combustible—(LC) requirement. The pass/fail criteria for materials such as wallboard and ceiling tile, which are either used to manufacture these spaces or will be exposed to the air flow, is controlled by the NFPA-255 and 259 tests; it is thus required to a “Limited Combustible” (LC) material to pass the 25/50/8 test. i.e. having a Flame Spread Rating of <25, a Smoke Developed Index of <50 per NFPA-255 and a Potential Heat Value of <3,500 Btu/lb (equal to 8,141 kJ/kg) per NFPA-259.
In response to the request of safer cables, manufacturers have introduced a new plenum cable with higher fire safety characteristics. This new classification of cable is called “Limited Combustible Cable” and is identified by the listing mark “Limited Combustible FHC 25/50 CMP”. To evaluate cable performances, it has appeared “logical” to apply the 25/50/8 requirements of LC materials for cables as tested per NFPA's 255 and 259. The primary difference between traditional combustible plenum cables and the limited combustible cable is that the latter is both insulated and jacketed with materials complying with the 25/50/8 requirements according to NFPA's 255 and 259.
Whichever is the fire safety characteristic which the plenum cables have to comply with (either NFPA 262 or “LC”), it is clear that a deep redesign of cable components, including materials for both primary insulation, cable fillers and jackets, has been found necessary.
The products of the present disclosure have alternatively been developed to support the possible adoption of a new NFPA standard referenced as NFPA 255 entitled “Limited Combustible Cables” with less than 50 as a maximum smoke index and NFPA 259 entitled “Heat of Combustion” which includes the use of an oxygen bomb calorimeter that allows for materials with less than 3,500 BTU/lb. for incorporation into cabling systems and buildings wherein survivability of the communication network from fires is required i.e. (military installation such as the Pentagon in Washington D.C.).
Table 2 provides a hierarchy of fire performance standards for North America and Europe.
TABLE 2Flammability Test Methods and Level of Severity for Wire and CableCable TypeTest MethodIgnition Source OutputDurationLimitedUL2424/NFPA8,141 KJ/kg (3,500 BTU/lb.)10 minCombustible259/255/UL723CMPSteiner Tunnel  88 kW (300k BTU/hr.)20 min.UL 910/NFPA 262CMRRISER 154 kW (527k BTU/hr.)30 min.UL 1666/UL2424/NFPA 259CPDSingle Burning Item  30 kW (102k BTU/hr.)30 min.Class D(20 min burner)CPDModified IEC 60332-3  30 kW (102k BTU/hr.)20 min.Class D(Backboard behind ladder (heatimpact))CMIEC 60332-320.5 kW (70k BTU/hr.)20 min.CMXVertical Tray20.5 kW (70k BTU/hr.)20 min.CMUCIEC 60332-1/ULVW-1Bunsen Burner 1 min.(15 sec. Flame)Cable Fire Performance (Levels of Severity)NFPA 255 & NFPA 259/LC/CPD Class B1+/UL 2424(most stringent)NFPA 262/EN 50289/FT-6/CPD Class B1/UL 910|UL 1666 Riser/FT-4/CPD Class C & B2|UL 1581 Tray/IEC 60332-3/FT-2/CPD Class D|VW 1/IEC 60332-1/FT-1/CPD Class E(least stringent)
There is thus a strong need in the art to provide for alternative foamable perfluoropolymer compositions useful as cable components, which can be easily processed in the melt using conventional equipments, able to comply with the limited combustible requirements and which possess outstanding electrical properties, making it suitable for so-called-augmented category 6 or 6a-cables” to be used in Gigabit Ethernet and future higher speed LAN applications.
This disclosure relates to improved materials that can be used as wire insulation, cable fillers (e.g. crosswebs) and cable jacketing for communication cables which are run through air plenums without the use of a metal conduit and which notably conform to Telecommunication Industry Association (TIA), Underwriters Laboratories (UL) and National Electrical Code standards.
For these applications requiring survivability from flame spread and smoke generation, the cellular products of the present disclosure will be an effective method for reducing material content and the fuel load of cables in such critical environments.